WEIMAR, Germany – President Barack Obama absorbed the stark horrors memorialized at the Buchenwald concentration camp yesterday and said the lesson for the modern world is vigilance against evil, against subjugation of the weak and against the “cruelty in ourselves.”

Obama honored the 56,000 who died at the Nazi camp and the thousands who survived. He invoked, too, his great-uncle, who helped liberate a Buchenwald satellite prison in 1945 and came back a haunted man.

“More than half a century later, our grief and our outrage over what happened have not diminished,” Obama said after witnessing the crematory ovens, barbed-wire fences, guard towers and the clock set at 3:15, marking the moment of the camp's liberation by the U.S. Army in the afternoon of April 11, 1945.

He challenged Iranian President Mahmoud Ahmadinejad, who has expressed doubts that 6 million Jews died at the hands of the Nazis, to visit, too.

“To this day, there are those who insist the Holocaust never happened,” Obama said. “This place is the ultimate rebuke to such thoughts, a reminder of our duty to confront those who would tell lies about our history.”

The president said he saw – reflected in the Nazi brutality against Jews and the other impounded outcasts – Israel's capacity to empathize with the suffering of others. He said that gave him more hope Israel and the Palestinians can achieve an equitable and lasting peace.

Toward that elusive goal, Obama is sending special envoy George Mitchell back to the Middle East next week. The president's outreach to Islam in his Cairo speech a day earlier was well-received in the Muslim world, and he is hoping the response will make progress more possible in the intractable dispute at the core of Muslim and Arab anger toward the West.

Israel, meanwhile, said it would not heed Obama's appeal to halt all settlement activity on lands the Palestinians claim for a future state. Officials said the government plans to allow construction inside existing West Bank settlements to accommodate growing families.

For Obama, the visit on a chilly, overcast day was a touchstone of his ancestry.

Obama's great-uncle, Charlie Payne, was among troops of the 89th Infantry Division who liberated a nearby subcamp, Ohrdruf, the same month.

“He returned from his service in a state of shock,” Obama said, “saying little and isolating himself for months on end from family and friends.” Payne bore “painful memories that would not leave his head.”

The president said Buchenwald “teaches us that we must be ever-vigilant about the spread of evil in our own time, that we must reject the false comfort that others' suffering is not our problem, and commit ourselves to resisting those who would subjugate others to serve their own interests.”

He added: “It's also important for us, I think, to remember that the perpetrators of such evil were human, as well, and that we have to guard against cruelty in ourselves.”

Obama was the first U.S. president to tour Buchenwald. In 2003, a tearful President George W. Bush visited the Auschwitz concentration camp in Poland, which his father saw in 1987 as vice president. Obama noted Dwight D. Eisenhower, then Allied commander and a future president, saw Ohrdruf and demanded that everything there be documented lest allied accounts of the atrocities be dismissed as propaganda.

Obama privately met several Buchenwald survivors on the eve of the 65th anniversary of the Allies' landing at Normandy, France, that led to the crushing of Nazi Germany. He toured the remains of the hillside compound with Chancellor Angela Merkel and Nobel Peace Prize winner Elie Wiesel, who was once a starving teenager in the camp.

The victims at Buchenwald included 11,000 Jews, but also communists, Gypsies and other minorities from across central Europe.

One by one, the president, Merkel, Wiesel and Buchenwald survivor Bertrand Herz placed pale yellow roses on a metal plaque known as the Living Memorial, kept permanently at body temperature as a monument to the victims.

Obama, in a dark suit and red tie, wore a torn red ribbon as a sign of mourning.

Huddled in conversation, he and the others walked between rows of barracks where inmates were worked to exhaustion before dying of starvation or disease, or were tortured in grisly experiments or lined up and shot.

Today, the barracks are just foundation and rubble – preserved as testimony.

Obama remarked on the contrast with the bucolic setting: rolling, wooded hills where power turbines turn gently in the wind.

Wiesel said to him, “If only these trees could talk.”

Wiesel spoke movingly about the death of his father a few months before the liberation of the camp, calling his journey there “a way of coming and visiting my father's grave.” He added, “But he had no grave. His grave is somewhere in the sky, which has become in those years the largest cemetery of the Jewish people.”

At the base of Buchenwald's hill, the four placed more roses at a monument to victims of Little Camp, Buchenwald's most notorious compound.

After the tour, Obama visited troops being treated at the Landstuhl U.S. military hospital for wounds suffered in the Iraq and Afghanistan wars. He then flew to Paris to reunite with his family, meet French President Nicolas Sarkozy today and commemorate the D-Day anniversary.

Last night, Michelle Obama and her two daughters paid a surprise visit to the Eiffel Tower, delighting tourists.

The first lady, 10-year-old Malia and 7-year-old Sasha spent about 45 minutes at the Parisian icon as Obama was arriving in France.

At Landstuhl, Obama pinned a Purple Heart medal on the chest of U.S. Army Cpl. Steven Baker, 24, who was wounded while on patrol in Afghanistan's Wardak province.